A reel lawnmower is a type of lawnmower that cuts grass using a rotating reel and a bed knife. The reel is a cylinder that has blades extending across its face, each blade being relatively sharp. The bed knife is a planar surface that is also relatively sharp. The reel is mounted on the lawnmower adjacent the bed knife with its longitudinal axis parallel to and just above the bed knife. As the reel lawnmower moves over the grass, the reel rotates about the longitudinal axis, and the reel blades intermittently pass in close proximity to bed knife. If the both the reel blade and the bed knife are relatively sharp, a scissoring effect results such that the top of the grass is cleanly snipped at a consistent height. Such grass cutting is sometimes referred to as turf manicuring.
Routine use may cause the blades on the reel to become worn. For example, FIG. 1 is a perspective view of a reel 101 of the prior art, and FIG. 2 is a partial side view of a reel blade 103 of the reel 101, illustrating a cutting edge 105 and a cutting surface 107 of the reel blade 103. If the cutting edge 105 becomes relatively dull, the reel lawnmower may rip or tear the grass instead of shearing the grass with a scissoring action. Ripping or tearing may reduce the appearance of the grass and may negatively affect the health of the grass plant, which may expend energy repairing the ripped surface instead of maintaining plant color and growth. Further, if the cutting surface 107 becomes relatively uneven, the reel lawnmower may cut at an inconsistent height, which is undesirable where turf manicuring is desired, such as on golf courses.
The problems associated with dull reel blades may be mitigated by periodically sharpening the reel blades 103. One manner of sharpening the reel blades 103 is backlapping, which involves applying a grinding compound while rotating the reel backward at a high speed. However, backlapping may be ineffective to sharpen the reel if the blades are excessively dull or have been nicked by rocks. In such cases, the effectiveness of backlapping may be restored by relief grinding the reel removing metal from an angled relief surface 109 on a back side of the reel blade 103 opposite from the cutting edge 105. The reduced cross-section along the angled relief surface 109 improves the ability of the grinding compound to flow between the reel and bed knife blades during backlapping. Thus, relief grinding is not effective to sharpen the cutting edges 105 or to level the cutting surfaces 107 of the reel blades 103; it merely improves the likelihood of sharpening the reel through subsequent backlapping.
Grinding processes that actually sharpen the cutting edges 105 and level the cutting surfaces 107 include single blade grinding and spin grinding. Both types of grinding involve moving a grinding wheel along the reel blade 103 to sharpen the cutting edge 105 while leveling the cutting surface 107. With single blade grinding, the grinding wheel is moved along an individual reel blade to sharpen its cutting edge 105 and level its cutting surface 107. When the blade is sharp, the reel is rotated and the next reel blade is sharpened. Thus, single blade grinding may be performed without removing the reel from the lawnmower. However, because each reel blade is sharpened independently, single blade grinding may distort the cylindrical shape of the reel, preventing the reel from cutting uniformly.
The reel may be uniformly sharpened by spin grinding the reel. Spin grinding involves contacting the reel with a rotating grinding wheel. The axis of rotation of the grinding wheel is oriented generally parallel to the axis of rotation of the reel, and both the grinding wheel and the reel are rotated at relatively high rotational velocities. One disadvantage of spin grinding is that vibrations result when the wheel and the reel contact each other while spinning at relatively high rotational velocities. The vibrations can be dampened, but the equipment required to perform the dampening makes the spin grinder heavy and expensive. For example, most spin grinding machines are large machines that are integrated into a cabinet or table, much like a table saw, and are normally housed indoors where they are powered using electricity. Because spin grinding machines may be cumbersome to move, the reel unit is normally removed from the lawnmower and transported to the location of the machine for sharpening.
Another disadvantage of the grinding processes summarized above is that these processes may be poorly suited for sharpening rotary lawnmower blades, which are planar blades having a sharp cutting surface on each half of the blade. For example, the spin grinding machine is not designed to accept the rotary lawnmower blade, and single blade grinding may result in relatively uneven sharpening. Additionally, single blade grinding is time-consuming, especially in cases in which multiple blades are to be sharpened. From the above, it is apparent that a need exists in the industry for an improved lawn blade sharpener.